12 ENVIRONMENT OF VERTEBRATE LIFE, ETC. 



scratched rock surfaces, beds of conglomerate composed of angular pebbles, 

 and the presence of large erratic blocks in marine deposits. 



The typical occurrences of such evidence in rocks of Permian and 

 Cambrian age are obvious enough, but more obscure evidence is difficult to 

 judge. Erratic blocks may be carried trapped in the roots of drifting trees 

 and deposited far from shore, or they may be carried by icebergs for enormous 

 distances. Angular conglomerates may result from landslides, rock glaciers, 

 or even sudden and violent floods originating on hillsides from cloudbursts. 

 Their interpretation when discovered must be cautiously approached. 

 Marks simulating glacial scratches may be produced by the slipping of 

 masses of rocks or even by the movements of sediments previous to their 

 cementation into rock.^ 



METAMORPHOSED SEDIMENTS. 



These present so many difficulties in their interpretation which must 

 be solved by severely technical methods that the problem must be in part 

 left to the specialist in petrography, but when the original nature of the 

 rocks is determined the history can be read upon the lines which have been 

 suggested above. No paleogeographer should neglect the important reve- 

 lations that may come from metamorphic rocks when completely and cor- 

 rectly interpreted. 



IGNEOUS ROCKS. 



Unless secondarily deposited by water or wind, igneous rocks affect 

 the problem only in an indirect way. The alteration of any sediments by 

 intrusion or burial by igneous rocks may aid in delimiting a unit, or the 

 stratigraphic relations to younger or older igneous rocks may be a determin- 

 ing factor. Changes in color and texture due to local metamorphism can 

 usually be easily detected. 



(b) Isolation of the Unit by Limiting Planes. 



A stratigraphic unit may be sharply set off from adjacent units by 

 structural breaks, by erosional breaks, by changes in the character of the 

 material, by changes in the bed alone; or it may pass so imperceptibly into 

 one or other of the adjacent beds that the line of separation is indistinguish- 

 able or only distinguishable by paleontological evidence. 



In the case of overthrusts, where older beds are forced above and across 

 younger beds, the line between the two is generally very clearly marked 

 both by the sudden change in the character of the material and contained 

 fossils (if present) and by the disturbance of the rocks accompanying the 



' Woodworth, J. B., Boulder Beds of the Caney Shales at Talahina, Oklahoma, Bull. Geol. 

 Soc. Amer., vol. 23, p. 462, 1912. 



